The present invention relates to a xcex2-keto ester compound, a xcex2-hydroxy acid compound and an acetonide form of a 1,3-diol derivative, which are useful as a synthetic intermediate for a pharmaceutical or agrichemical agent, and production methods thereof. The xcex2-keto ester compound, the xcex2-hydroxy acid compound and the acetonide form of a 1,3-diol derivative obtained by the present invention are useful as a synthetic intermediates for an epothilone derivative being developed as a pharmaceutical agent having antitumor activity.
Epothilone is a substance produced by myxobacterium Sorangium cellulosum and is known to have high antitumor activity. 
In recent years, energetic studies of synthetic structural conversion in an effort to obtain an epothilone derivative showing higher performance are ongoing (general synthetic methods of epothilone are shown in J. Am. Chem. Soc. 2001, 123, 5407-5413 and publications quoted in this reference). For this purpose, various compounds useful as synthetic intermediates therefore have been studied.
The present inventors have investigated synthetic intermediates useful for the production of the above-mentioned epothilone derivative and noted the idea that the structure of a novel compound, tert-butyl 4-cyano-4-methyl-3-oxopentanoate, represented by the following formula 
xe2x80x83is useful as the above-mentioned synthetic intermediate. However, this novel compound is difficult to synthesize. That is, as a compound usable as a material for synthesizing the tert-butyl 4-cyano-4-methyl-3-oxopentanoate, a conventionally known 2-cyano-2,2-dimethylacetate represented by the following formula 
xe2x80x83wherein R1 is alkyl group having 1 to 6 carbon atoms, is considered, but as is clear from the above-mentioned structural formula, 2-cyano-2,2-dimethylacetate has cyano group and ester residue as reaction sites in a single molecule. Thereby making selective production of the above-mentioned tert-butyl 4-cyano-4-methyl-3-oxopentanoate seems difficult.
However, if the above-mentioned tert-butyl 4-cyano-4-methyl-3-oxopentanoate can be synthesized from the above-mentioned 2-cyano-2,2-dimethylacetate, this compound is a desirable starting material because the compound can be economically prepared on a large-scale and in short steps. Thus, a development of a method for the production of the above-mentioned novel tert-butyl 4-cyano-4-methyl-3-oxopentanoate from 2-cyano-2,2-dimethylacetate has been investigated.
The compounds expected to be usable as synthetic intermediates for an epothilone derivative include a novel compound not published heretofore, which is tert-butyl 4-cyano-4-methyl-3-hydroxypentanoate represented by the following formula 
xe2x80x834-cyano-3-hydroxy-4-methylpentanoic acid represented by the following formula 
xe2x80x83optically active 4-cyano-3-hydroxy-4-methylpentanoic acid represented by the following formula 
xe2x80x83and optically active 4-cyano-3-hydroxy-4-methylpentanoate of the following formula 
xe2x80x83wherein R3 is alkyl group having 1 to 6 carbon atoms. These synthetic intermediates are novel compounds, and convenient and economical production of these synthetic intermediates is expected to afford a large-scale synthesis of the final product, an epothilone derivative.
Accordingly, an object of the present invention is to provide a novel synthetic intermediate for the synthesis of an epothilone derivative useful as a pharmaceutical or agrichemical agent, particularly as an antitumor agent, and production methods thereof.
[1] A xcex2-keto ester compound represented by the following formula 
xe2x80x83wherein R4 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, or an optically active form thereof.
[2] The compound of the above-mentioned [1], wherein R4 is hydrogen atom, or an optically active form thereof.
[3] The compound of the above-mentioned [1], wherein R4 is alkyl group having 1 to 6 carbon atoms, or an optically active form thereof.
[4] A production method of a xcex2-keto ester compound of the above-mentioned [1], which comprises condensation of a 2-cyano-2,2-dimethylacetate represented by the following formula 
xe2x80x83wherein R1 is alkyl group having 1 to 6 carbon atoms, with an alkyl ester represented by the following formula 
xe2x80x83wherein R4 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, in the presence of a strong base.
[5] The production method of the above-mentioned [4], wherein R4 is hydrogen atom.
[6] The production method of the above-mentioned [4], wherein R4 is alkyl group having 1 to 6 carbon atoms.
[7] The production method of the above-mentioned [4], comprising adding lithium diisopropylamide as a strong base to a mixture of 2-cyano-2,2-dimethylacetate represented by the formula (II) and an alkyl ester represented by the formula (III).
[8] The production method of any of the above-mentioned [4]-[7], which comprises adding cyanoacetate of the following formula 
xe2x80x83wherein R1 is alkyl group having 1 to 6 carbon atoms, and dimethyl sulfate continuously or discontinuously to a sodium hydride-containing tetrahydrofuran solution to give 2-cyano-2,2-dimethylacetate of the formula (II), and condensation thereof with an alkyl ester of the formula (III).
[9] A xcex2-hydroxy acid compound represented by the following formula 
xe2x80x83wherein R2 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, and R4 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, provided that when R4 is alkyl group having 1 to 6 carbon atoms, R2 should be tert-butyl group, an optically active form thereof or a salt thereof.
[10] The compound of the above-mentioned [9], wherein R4 is hydrogen atom, an optically active form thereof or a salt thereof.
[11] The compound of the above-mentioned [9], wherein R4 is alkyl group having 1 to 6 carbon atoms and R2 is tert-butyl group, an optically active form thereof or a salt thereof.
[12] A production method of a xcex2-hydroxy acid compound represented by the formula (V) of the above-mentioned [9], which comprises reducing a xcex2-keto ester compound represented by the following formula 
xe2x80x83wherein R2 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, and R4 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, provided that when R4 is alkyl group having 1 to 6 carbon atoms, R2 should be tert-butyl group, or a salt thereof.
[13] The production method of the above-mentioned [12], wherein R4 is alkyl group having 1 to 6 carbon atoms and R2 is tert-butyl group.
[14] The production method of the above-mentioned [13], wherein the reduction is carried out using alkali borohydride and divalent metal chloride.
[15] The production method of the above-mentioned [12], wherein R4 is hydrogen atom.
[16] The production method of the above-mentioned [15], wherein R2 is alkyl group having 1 to 6 carbon atoms.
[17] The production method of the above-mentioned [16], wherein the alkyl group having 1 to 6 carbon atoms is tert-butyl group.
[18] The production method of any of the above-mentioned [15]-[17], wherein the reduction is carried out using sodium borohydride.
[19] The production method of the above-mentioned [14], wherein the xcex2-keto ester compound of the formula (Ixe2x80x2), wherein R2 is alkyl group having 1 to 6 carbon atoms and R4 is hydrogen atom, is reduced to give a xcex2-hydroxy acid compound (V-4) represented by the following formula 
xe2x80x83wherein R2xe2x80x2 is alkyl group having 1 to 6 carbon atoms, which is a compound of the formula (V) wherein R2 is alkyl group having 1 to 6 carbon atoms and R1 is hydrogen atom, and the xcex2-hydroxy acid compound (V-4) is subjected to alkali hydrolysis to give a xcex2-hydroxy acid compound (V-5) of the following formula 
xe2x80x83which is a compound of the formula (V) wherein R2 and R4 are hydrogen atoms.
[20] The production method of the above-mentioned [19], wherein the resulting xcex2-hydroxy acid compound (V-5) is optically resolved to give a xcex2-hydroxy acid compound (V-6) of the following formula 
xe2x80x83or a salt thereof, which is then esterified with an alkylating agent to give a xcex2-hydroxy acid compound (V-7) represented by the following formula 
xe2x80x83wherein R3 is alkyl group having 1 to 6 carbon atoms, which is an optically active compound of the formula (V) wherein R2 is alkyl group having 1 to 6 carbon atoms and R4 is hydrogen atom.
[21] The production method of the above-mentioned [19] or [20], wherein each alkyl group having 1 to 6 carbon atoms is tert-butyl group.
[22] A production method of a xcex2-hydroxy acid compound (V-6) represented by the following formula 
xe2x80x83or a salt thereof, which is an optically active compound represented by the formula (V) of the above-mentioned [9], wherein R2 and R4 are hydrogen atoms, which comprises optical resolution of the xcex2-hydroxy acid compound (V-5) represented by the following formula 
xe2x80x83[23] The production method of the above-mentioned [22], wherein the xcex2-hydroxy acid compound (V-5) is converted to a salt with an optically active amine compound and optically resolved.
[24] A production method of a xcex2-hydroxy acid compound (V-7) represented by the following formula 
xe2x80x83wherein R3 is alkyl group having 1 to 6 carbon atoms, which is an optically active compound represented by the formula (V) of the above-mentioned [9], wherein R2 is alkyl group having 1 to 6 carbon atoms and R4 is hydrogen atom, which comprises esterification of a xcex2-hydroxy acid compound (V-6) represented by the following formula 
xe2x80x83or a salt thereof with an alkylating agent.
[25] An acetonide form of a 1,3-diol derivative, which is represented by the following formula 
xe2x80x83or an optically active form thereof.
[26] A production method of an acetonide form of a 1,3-diol derivative represented by the formula (VIII) of the above-mentioned [25], or an optically active form thereof, which comprises conversion of a 1,3-diol derivative represented by the following formula 
xe2x80x83or an optically active form thereof, to an acetonide form thereof.
[27] The production method of the above-mentioned [26], wherein the xcex2-hydroxy acid compound of the following formula 
xe2x80x83wherein R2 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, or an optically active form thereof is reduced to give a 1,3-diol derivative of the formula (VII) or an optically active form thereof, which is converted to an acetonide form.
[28] A production method of a 1,3-diol derivative represented by the following formula 
xe2x80x83or an optically active form thereof, which comprises reducing a xcex2-hydroxy acid compound represented by the following formula 
xe2x80x83wherein R2 is hydrogen atom or alkyl group having 1 to 6 carbon atoms, or an optically active form thereof.
The definition of each symbol is explained in the following.
In the present specification, alkyl group having 1 to 6 carbon atoms may be linear or branched and is exemplified by methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, tert-butyl group, pentyl group and hexyl group. R1 is preferably a group showing small steric hindrance, and is particularly preferably methyl group or ethyl group. R1, R1 and R3 are preferably methyl group or ethyl group in view of easy availability.
To achieve the above-mentioned object, the present invention has the following characteristics.
xcex2-keto Ester Compound (I) and Production Method Thereof
According to the production method of the present invention, as shown in the following Scheme 1,2-cyano-2,2-dimethylacetate represented by the formula (II) (hereinafter sometimes to be referred to as 2-cyano-2,2 dimethylacetate (II)) and an alkyl ester represented by the formula (III) (hereinafter sometimes to be referred to as alkyl ester (III)). Are condensed to give a novel xcex2-keto ester compound represented by the formula (I) (hereinafter sometimes to be referred to as xcex2-keto ester compound (I)). 
In the above-mentioned Scheme, R1 is alkyl group having 1 to 6 carbon atoms and R4 is hydrogen atom or alkyl group having 1 to 6 carbon atoms.
The production method of the present invention is realized by, for example, adding a strong base to a mixture of 2-cyano-2,2-dimethylacetate (II) and an alkyl ester (III). By carrying out the reaction by adding (e.g., dropwise addition) a strong base to the above-mentioned mixture, self condensation during anionizing the alkyl ester (III) can be suppressed, whereby 2-cyano-2,2-dimethylacetate (II) and an alkyl ester (III) are efficiently condensed. As a result, the above-mentioned xcex2-keto ester compound (I) can be obtained in a high yield.
As the strong base to be used for the production method of the present invention, for example, lithium diisopropylamide (LDA), lithium hexamethyldisilazide, lithium isopropylcyclohexylamide, lithium dicyclohexylamide, lithium 2,2,6,6-tetramethylpiperidide, bromo magnesium diisopropylamide and the like can be used, of which LDA is preferable.
The solvent to be used for the preparation of a mixture of 2-cyano-2,2-dimethylacetate (II) and the above-mentioned alkyl ester (III) is subject to no particular limitation as long as it is inert to the above-mentioned condensation. Examples of the inert solvent include hydrocarbon solvents such as toluene, hexane and the like, ether solvents such as tetrahydrofuran (THF) and the like, and a mixed solvent thereof.
While the amount of the solvent to be used for preparing the mixture is not particularly limited, it is preferably a 0.5- to 10-fold weight, more preferably a 1- to 5-fold weight, relative to the total weight of the above-mentioned 2-cyano-2,2-dimethylacetate (II) and an alkyl ester (III). When the amount of use of the solvent is less than 0.5-fold amount relative to the amount of the reagent, the viscosity becomes high and the stirring may not be done sufficiently. When the amount of use of the solvent exceeds 10-fold amount of the reagent, it causes lower volume efficiency, which may not be economical.
The amount of the strong base to be used is not particularly limited and varies depending on the kind of the strong base. When, for example, LDA is used as a strong base, it is preferably used in an amount of 0.95-1.5 mol, more preferably 1.0-1.3 mol, per 1 mol of 2-cyano-2,2-dimethylacetate (II). When the amount of use of the LDA is less than 0.95 mol per 1 mol of 2-cyano-2,2-dimethylacetate (II), the reaction stops on the way to often reduce the yield. When the amount of use of the LDA exceeds 1.5 mol per 1 mol of 2-cyano-2,2-dimethylacetate (II), the amount of by-products tends to increase.
When LDA is used as a strong base, it may be used in the form of a solution upon dissolution of LDA in a solvent such as heptane, THF, hexane, toluene and the like. By forming such LDA solution, LDA becomes stable and easy to handle.
When LDA is used as a strong base, the temperature, at which the reaction is carried out, is preferably not higher than xe2x88x9250xc2x0 C., more preferably from xe2x88x92100xc2x0 C. to xe2x88x9250xc2x0 C., particularly preferably around xe2x88x9270xc2x0 C. (xe2x88x9280xc2x0 C. to xe2x88x9260xc2x0 C.), for the production of xcex2-keto ester compound (I) in a higher yield. When the above-mentioned reaction is carried out at a temperature around xe2x88x9270xc2x0 C. using LDA, self condensation of the above-mentioned alkyl ester (III) is further suppressed to permit extremely efficient progress of the reaction to quantitatively produce the objective xcex2-keto ester compound (I) (yield: not less than 90%) using 1 equivalent amount each of an alkyl ester (III), 2-cyano-2,2-dimethylacetate (II) and LDA.
After the above-mentioned reaction shown in Scheme 1, the reaction mixture is warmed to a temperature of from xe2x88x9210xc2x0 C. to room temperature, which is followed by addition of an acidic aqueous solution such as hydrochloric acid, acetic acid and the like, or addition of the reaction mixture into an acidic aqueous solution, to desirably make the solution near neutral. Thereafter, a generally used solvent, such as the above solvent used, or ethyl acetate or toluene is used to extract the resulting product.
Through the production method shown in Scheme 1 above, a xcex2-keto ester compound (I) of the following formula 
xe2x80x83wherein R4 is as defined above, can be produced in a yield of not less than 90%.
The following formula 
xe2x80x83wherein R4xe2x80x2 is alkyl group having 1 to 6 carbon atoms, which is the xcex2-keto ester compound (I) wherein R4 is alkyl group (R4xe2x80x2) having 1 to 6 carbon atoms, includes two stereoisomers of the formula 
xe2x80x83wherein R4xe2x80x2 is alkyl group having 1 to 6 carbon atoms, and the formula 
xe2x80x83wherein R4xe2x80x2 is alkyl group having 1 to 6 carbon atoms.
The xcex2-keto ester compound (I) is a novel compound found for the first time by the present inventors. This compound is considered to be very useful as a synthetic intermediate for the production of an epothilone derivative being developed as a pharmaceutical agent having antitumor activity.
The aforementioned production method is only one example of a method for producing xcex2-keto ester compound (I), and while the xcex2-keto ester compound (I) of the present invention is not limited to those produced by the above-mentioned production method, it is preferably those produced by the above-mentioned production method.
The xcex2-keto ester compound (I) of the present invention is easy to structurally enolize. The enol may not be susceptible to reduction, in which case the use of the xcex2-keto ester compound (I) as a synthetic intermediate may cause inconvenience in the production of an xcex2-hydroxy acid derivative (V). In the production method of the present invention, the proportion of enol form after treatment by the reaction shown in Scheme 1 is preferably reduced as much as possible to eliminate the possibility of the above-mentioned inconvenience.
When pH of the aqueous layer was changed from near neutral to 2 in the water washing treatment of the xcex2-keto ester compound, it was found that pH 6.5-7.5 was optimal for suppressing the generation of enol, by observation of the shift of peaks in 1H-NMR.
The 2-cyano-2,2-dimethylacetate (II) and an alkyl ester (III) used as starting materials for the production method of xcex2-keto ester compound (I) of the present invention are respectively obtained by conventionally known methods. It is also possible to use commercially available ones, but 2-cyano-2,2-dimethylacetate (II) is preferably produced by the following production method proposed by the present inventors.
Production Method of 2-cyano-2,2-dimethylacetate (II)
The 2-cyano-2,2-dimethylacetate (II) used as a starting material for the production method of xcex2-keto ester compound (I) of the present invention is, as mentioned above, a conventionally known compound. The present inventors have found a production method shown in the following Scheme 2 that affords 2-cyano-2,2-dimethylacetate (II) in a high yield. 
In this Scheme, R1 is as defined above.
That is, in the present invention, a novel production method of 2-cyano-2,2-dimethylacetate (II) is also provided, which comprises continuously or discontinuously adding cyanoacetate represented by the above-mentioned formula (IV) (hereinafter to be sometimes referred to as cyanoacetate (IV)) and dimethyl sulfate to a sodium hydride (NaH)-containing THF solution.
In the above-mentioned production method, the use of NaH as a base permits a quantitative reaction as compared to the use of other bases, because it produces less contaminant. Furthermore, the use of THF as a solvent suppresses mixing of the resulting product in a distillation solvent during concentration of the reaction mixture after completion of the reaction, which in turn leads to a higher yield as compared to the use of other solvents. When R1 is ethyl group (when ethyl cyanoacetate is used as cyanoacetate (IV)), mixing of the product in a distillation solvent during the above-mentioned concentration of the reaction mixture can be preferably suppressed further.
Sodium hydride is generally used as a dispersion in a 60% mineral oil. The amount of sodium hydride to be used is generally 2-3 mol, preferably 2.1-2.5 mol, per 1 mol of cyanoacetate (IV). The amount of THF to be used is generally 2-6 parts by weight, preferably 3-5 parts by weight, per 1 part by weight of ethyl cyanoacetate.
In the production method of the 2-cyano-2,2-dimethylacetate (II), cyanoacetate (IV) and dimethyl sulfate may be added continuously or discontinuously to an NaH-containing THF solution. It is preferable to add them in such a manner as to make the amount of hydrogen produced during the reaction below excess and to facilitate control of heat generation in the course of the reaction. The proportions of cyanoacetate (IV) and dimethyl sulfate to be added to an NaH-containing THF solution can be made to be almost the same as the proportions of cyanoacetate (IV) and dimethyl sulfate used for the reaction. When they are added continuously, the entire amount of cyanoacetate (IV) and dimethyl sulfate to be used for the reaction is respectively added continuously. When they are added discontinuously, the entire amount of cyanoacetate (IV) and dimethyl sulfate to be used for the reaction is divided in plural portions, and added discontinuously. In the latter case, the number of division of cyanoacetate (IV) is preferably almost the same as that of dimethyl sulfate. The proportions of cyanoacetate (IV) and dimethyl sulfate to be added to the NaH-containing THF solution can be made almost the same as the proportions of the cyanoacetate (IV) and dimethyl sulfate to be used for the reaction. This number of division varies depending on the scale of the reaction, and is free of any particular limitation as long as the reaction heat can be removed and the reaction temperature range to be mentioned later can be maintained. Where necessary, cyanoacetate (IV) and dimethyl sulfate may be mixed for dropwise addition.
The molar ratio of the cyanoacetate (IV) and dimethyl sulfate for the addition mentioned above is preferably almost 1:2 (1:2.0-2.5). The temperature of the above-mentioned addition is preferably 20-60xc2x0 C., more preferably 35-45xc2x0 C. When the above-mentioned temperature is lower than 20xc2x0 C., the reaction becomes late and may proceed beyond control due to sudden heat generation caused by the reaction. When the above-mentioned temperature exceeds 60xc2x0 C., the yield may become lower due to the side reaction.
By adding cyanoacetate (IV) and dimethyl sulfate to an NaH-containing THF solution as mentioned above at the aforementioned molar ratio and temperature, generation of hydrogen and heat due to the reaction can be controlled by the rate of addition of cyanoacetate (IV) and dimethyl sulfate. The reaction mixture under such conditions becomes a slurry having a relatively lower viscosity, where sodium monomethylsulfate has precipitated out.
In the above-mentioned production method, both dropwise addition or in-flowing may be employed. The time necessary for the addition is free of any particular limitation as long as the reaction heat can be removed, the amount of hydrogen generated is below excess and the temperature stays within the range to be mentioned below, and varies depending on the scale of the reaction.
In the production method of the above-mentioned 2-cyano-2,2-dimethylacetate (II), a part of either cyanoacetate (IV) or dimethyl sulfate may be charged in advance in an NaH-containing THF solution. In this case, cyanoacetate (IV) is preferably charged in advance in a preferable proportion of 5 wt %-20 wt %, more preferably 5 wt %-10 wt %, of the entire amount to be used for the reaction.
After the reaction shown in the above-mentioned Scheme 2, the reaction mixture is treated with, for example, a dilute aqueous acetic acid solution, a THF layer is extracted, THF is evaporated at the atmospheric pressurexe2x80x94under somewhat reduced pressure (20-26.6 kPa), and then evaporated under reduced pressure of 1.3-1.6 kPa, whereby 2-cyano-2,2-dimethylacetate (II) can be obtained at 62-69xc2x0 C.
By the production method shown in the above-mentioned Scheme 2,2-cyano-2,2-dimethylacetate (II) can be produced in a yield of not less than 80%, particularly not less than 85%.
The xcex2-keto ester compound (I) of the present invention is preferably obtained economically in a high yield by obtaining 2-cyano-2,2-dimethylacetate (II) from the reaction of cyanoacetate (IV) with dimethyl sulfate according to the method shown in the aforementioned Scheme 2, and then reacting 2-cyano-2,2-dimethylacetate (II) with an alkyl ester (III) according to the method shown in the aforementioned Scheme 1. As mentioned above, it is particularly preferable that R1 be methyl group or ethyl group and the method of Scheme 1 be realized by dropwise addition of a lithium diisopropylamide solution to a mixture of ethyl 2,2-dimethylcyanoacetate and tert-butyl acetate.
xcex2-Hydroxy Acid Compound (V) and Production Method Thereof
In the present invention, a production method of a novel xcex2-hydroxy acid compound (V) of the following formula 
xe2x80x83wherein R2 and R4 are as mentioned above, provided that when R4 is alkyl group having 1 to 6 carbon atoms, R2 should be tert-butyl group, is also provided, which comprises reducing a xcex2-keto ester compound of the following formula 
xe2x80x83wherein R2 and R4 are as mentioned above, provided that when R4 is alkyl group having 1 to 6 carbon atoms, R2 should be tert-butyl group (hereinafter sometimes to be referred to as xcex2-keto ester compound (Ixe2x80x2)) or a salt thereof.
The production method of xcex2-hydroxy acid compound (V) of the present invention comprises adding a reducing agent to xcex2-keto ester compound (Ixe2x80x2) to allow reaction.
The reducing agent to be used for the reduction may be any as long as it can reduce xcex2-keto ester compound (Ixe2x80x2) to xcex2-hydroxy acid compound (V) (one capable of reducing oxo(ketone) group to hydroxy group), which is exemplified by alkali borohydride such as sodium borohydride, lithium borohydride and the like, diisobutylaluminum hydride and the like. Of these, alkali borohydride, particularly sodium borohydride, is preferable because the objective compound can be obtained quantitatively.
The amount of the reducing agent to be added is free of any particular limitation, but is preferably 0.25-1.0 mol, more preferably 0.3-0.7 mol, per 1 mol of the xcex2-keto ester compound (Ixe2x80x2). When the amount of the reducing agent to be added is less than 0.25 mol per 1 mol of the xcex2-keto ester compound (Ixe2x80x2), the reaction cannot be completed and the yield tends to be lower. When the amount of the reducing agent to be added exceeds 1.0 mol per 1 mol of the xcex2-keto ester compound (Ixe2x80x2), it may lead to an economical burden.
In this reduction reaction, the reaction temperature varies depending on the starting materials. When, for example, a xcex2-keto ester compound of the following formula 
xe2x80x83wherein R4xe2x80x2 is as defined above, which is a compound of the above-mentioned formula (Ixe2x80x2) wherein R4 is alkyl group (R4xe2x80x2) having 1 to 6 carbon atoms and R2 is tert-butyl group, is used as a starting material, it is preferably from 0xc2x0 C. to 30xc2x0 C., because the reaction speed and stereoselectivity can be maintained high. When the reaction temperature is lower than 0xc2x0 C., the reaction speed may become slow and the reaction time may be prolonged. When the reaction temperature exceeds 30xc2x0 C., stereoselectivity may become lower. The reaction time in this case is preferably 0.25-10 hr. Moreover, when a xcex2-keto ester compound of the following formula 
xe2x80x83which is a compound of the above-mentioned formula (Ixe2x80x2) wherein R4 is hydrogen atom and R2 is tert-butyl group, is used as a starting material, the reaction temperature is generally 0-50xc2x0 C., preferably 10-30xc2x0 C., and the reaction time may be any as long as the reaction heat can be controlled, which is preferably 2-10 hr.
The solvent to be used for the above-mentioned reduction may be any as long as it is inert to the above-mentioned reducing agent and can be selected as appropriate depending on the reducing agent to be used. For example, when sodium borohydride is used as a reducing agent, the usable solvent includes water, alcohol solvents (e.g., methanol, ethanol, 2-propanol), ester solvents, ether solvents (e.g., tetrahydrofuran (THF), dioxane), and mixed solvents thereof. Of these, alcohol solvents are preferable because stereoselectivity and reaction speed can be high and by-product is produced only in a smaller amount. In addition, when diisobutylaluminum hydride, which requires an aprotic solvent, is used as a reducing agent, ether solvents (e.g., THF, ether), hydrocarbon solvents (e.g., toluene, hexane, cyclohexane) and the like can be used, of which dry THF is preferably used.
While the amount of the above-mentioned solvent to be used is free of any particular limitation, it is preferably from a 0.5-fold amount to a 10-fold amount, more preferably from a 1-fold weight to a 2-fold weight, relative to the weight of the xcex2-keto ester compound (Ixe2x80x2). When the amount of the above-mentioned solvent to be used is less than 0.5-fold amount relative to the xcex2-keto ester compound (Ixe2x80x2), stirring does not proceed smoothly and uniform progress of the reaction may be prevented. When the amount of the above-mentioned solvent to be used exceeds a 10-fold amount relative to the xcex2-keto ester compound (Ixe2x80x2), the volume efficiency may become low, which is uneconomical.
It is also possible to reduce xcex2-keto ester compound (Ixe2x80x2) into xcex2-hydroxy acid compound (V) by catalytic reduction.
The xcex2-keto ester compound (Ixe2x80x2) to be used for this reaction can be produced by a method the same as or similar to the production method of the aforementioned xcex2-keto ester compound (I).
In this way, xcex2-hydroxy acid compound (V) of the following formula (V) 
xe2x80x83wherein R2 and R4 are as defined above, provided that when R4 is alkyl group (R4xe2x80x2) having 1 to 6 carbon atoms, R2 should be tert-butyl group, can be produced.
A compound of the following formula 
xe2x80x83wherein R4xe2x80x2 is alkyl group having 1 to 6 carbon atoms, which is the above-mentioned xcex2-hydroxy acid compound (V) wherein R4 is alkyl group having 1 to 6 carbon atoms and R2 is tert-butyl group, encompasses the following four optical isomers 
xe2x80x83wherein R4 is as defined above.
A compound of the following formula (V-2) 
xe2x80x83wherein R1 is as defined above, which is a xcex2-hydroxy acid compound (V) wherein R4 is hydrogen atom, encompasses the following two optical isomers 
xe2x80x83wherein R2 is as defined above.
The xcex2-hydroxy acid compound (V) of the present invention encompasses an optically active form, mixtures thereof (racemate, enantiomer mixture, diastereomer mixture) and the like. Furthermore, xcex2-hydroxy acid compound of the following formula (V-3) 
xe2x80x83wherein R4 is as defined above, which is the above-mentioned xcex2-hydroxy acid compound (V) wherein R2 is hydrogen atom, can form a salt. The xcex2-hydroxy acid compound (V) of the present invention also encompasses a salt form. Examples of the salt include alkali metal salts such as sodium, potassium etc., organic amine salts such as triethylamine salt etc., and the like.
This xcex2-hydroxy acid compound (V) is also a novel compound found for the first time by the present inventors and extremely useful as a synthetic intermediate for the production of an epothilone derivative under development as a pharmaceutical agent having antitumor activity.
The aforementioned production method is merely an example of the method for producing xcex2-hydroxy acid compound (V), and xcex2-hydroxy acid compound (V) of the present invention is not limited to those produced by the above-mentioned production method. However, it is preferably produced by the above-mentioned production method.
The present inventors have further found that, when alkali borohydride is used as a reducing agent in the production method of the above-mentioned xcex2-hydroxy acid compound (V), the ratio of stereo isomers of the resulting xcex2-hydroxy acid compound (V) can be changed by the presence or otherwise of a divalent metal salt during the reduction reaction.
That is, taking the above-mentioned xcex2-hydroxy acid compound of the following formula 
xe2x80x83wherein R4xe2x80x2 is alkyl group having 1 to 6 carbon atoms, which is a compound of the above-mentioned formula (V) wherein 4 is alkyl group (R4xe2x80x2) having 1 to 6 carbon atoms and R2 is tert-butyl group, (hereinafter sometimes to be referred to as xcex2-hydroxy acid compound (V-1)) as an example, a threo form isomers 
xe2x80x83are produced according to the production method of the above-mentioned xcex2-hydroxy acid compound (V). According to the present invention, the ratio of the resulting threo form and erythro form changes depending on the presence/absence of a divalent metal chloride during the above-mentioned reaction.
Specifically, when a divalent metal chloride is absent during the above-mentioned reduction, the ratio of the resulting threo form and erythro form is threo form:erythro form=91.9:8.1-90:10, and when the above-mentioned metal chloride is present, the ratio of the resulting threo form and erythro form is threo form:erythro form=68.8:31.2-46.7:53.3.
As the divalent metal chloride to be used in the present invention, for example, manganese(II) chloride (MnCl2), calcium chloride (CaCl2), zinc(II) chloride (ZnCl2) and the like are exemplified, wherein the use of manganese(II) chloride or calcium chloride is preferable, in view of the low reactivity of zinc(II) chloride.
When the above-mentioned metal chloride is used, the amount thereof to be added is free of any limitation and can be determined appropriately according to the reactivity of the xcex2-keto ester compound (Ixe2x80x2). Because xcex2-keto ester compound (Ixe2x80x2) is highly likely bonded coordinately with the metal chloride at a ratio of 1:1, the amount is preferably 1-3 mol, more preferably 1.5-2 mol, relative to 1 mol of the xcex2-keto ester compound (Ixe2x80x2). When the amount of the above-mentioned metal chloride to be added is less than 1 mol relative to 1 mol of the xcex2-keto ester compound (Ixe2x80x2), the ideal stereoselectivity is highly likely not achieved. When the amount of the metal chloride to be added exceeds 3 mol relative to 1 mol of the xcex2-keto ester compound (Ixe2x80x2), an economical burden may increase.
The xcex2-hydroxy acid compound (V) can be isolated and purified by subjecting the reaction mixture after the above-mentioned reduction to typical treatment and separation. For example, an organic solvent is added to the reaction mixture and the mixture is stirred and partitioned to remove the organic layer, the pH of the aqueous layer is adjusted, the aqueous layer is subjected to extraction with a solvent and the extract is concentrated to isolate the above-mentioned hydroxy acid compound (V).
The organic solvent to be added to the reaction mixture after reduction is exemplified by heptane, toluene, ethyl acetate, MIBK (methyl isobutyl ketone) and the like, with preference given to toluene in view of the low affinity for water and easy extraction. The time for stirring upon addition of the organic solvent is not particularly limited as long as the organic layer and the aqueous layer are sufficiently separated. The pH of the aqueous layer is adjusted to an acidic one (generally 1-3, preferably 1.5-2.5) using an acid such as hydrochloric acid, sulfuric acid and the like.
The above-mentioned solvent used for extraction of the objective xcex2-hydroxy acid compound (V) from the aqueous layer is free of any particular limitation and is exemplified by ethyl acetate, a mixed solution of ethyl acetate-n-butanol (1:1), toluene and the like, with preference given to toluene that resists mixing of water. Removal of the solvent (the above-mentioned alcohol solvent and the like) under reduced pressure before extraction facilitates, extraction with toluene.
The a xcex2-hydroxy acid compound (V-2) of the following formula 
xe2x80x83which is a xcex2-hydroxy acid compound (V) wherein R4 is hydrogen atom, is exemplified by
(1) the above-mentioned xcex2-hydroxy acid compound of the following formula 
xe2x80x83wherein R2xe2x80x2 is alkyl group having 1 to 6 carbon atoms, (hereinafter sometimes to be referred to as xcex2-hydroxy acid compound (V-4)), which is a xcex2-hydroxy acid compound (V) wherein R2 is alkyl group (R2xe2x80x2) having 1 to 6 carbon atoms and R4 is hydrogen atom,
(2) xcex2-hydroxy acid compound of the following formula 
xe2x80x83(hereinafter sometimes to be referred to as xcex2-hydroxy acid compound (V-5)), which is a xcex2-hydroxy acid compound (V) wherein R2 and R4 are both hydrogen atoms,
(3) xcex2-hydroxy acid compound of the following formula 
xe2x80x83(hereinafter sometimes to be referred to as xcex2-hydroxy acid compound (V-6)), which is an optically active form of the above-mentioned xcex2-hydroxy acid compound (V-5), and
(4) xcex2-hydroxy acid compound of the following formula 
xe2x80x83wherein R3 is alkyl group having 1 to 6 carbon atoms, (hereinafter sometimes to be referred to as xcex2-hydroxy acid compound (V-7)), which is an ester of the above-mentioned xcex2-hydroxy acid compound (V-6).
The xcex2-hydroxy acid compounds (V-5) to (V-7) can be produced by the aforementioned production method of the xcex2-hydroxy acid compound (V), and also by the following method.
Production Method of xcex2-hydroxy Acid Compound (V-5) Using xcex2-hydroxy Acid Compound (V-4) as a Starting Material
The xcex2-hydroxy acid compound (V-5) (4-cyano-3-hydroxy-4-methylpentanoic acid) is also a novel compound, and can be obtained by alkali hydrolysis of xcex2-hydroxy acid compound (V-4) as shown in the following Scheme 4. 
In the above-mentioned Scheme 4, R3xe2x80x2 is as defined above.
The xcex2-hydroxy acid compound (V-4) can be produced according to the production method of the aforementioned xcex2-hydroxy acid compound (V) (hereinafter sometimes to be referred to as Step 1). Therefore, alkali hydrolysis of xcex2-hydroxy acid compound (V-4) (hereinafter sometimes to be referred to as Step 2) is preferably performed successively from the production method of the xcex2-hydroxy acid compound (V) to reduce the number of steps. To be specific, xcex2-hydroxy acid compound (V-5) can be obtained without isolation of xcex2-hydroxy acid compound (V-4), by dropwise addition of an alkali solution to a solution containing a reducing agent in the above-mentioned step 1. Alternatively, water is added to a solution containing a reducing agent in the above-mentioned step 1, to separate the inorganic compound resulting from the reducing agent, which compound is subjected to alkali hydrolysis to give xcex2-hydroxy acid compound (V-5).
The above-mentioned alkali solution may be any as long as it can hydrolyze an alkyl group of the ester to give carboxyl group. Examples thereof include aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, aqueous sodium carbonate solution, aqueous potassium carbonate solution and the like. Of these, 10% aqueous sodium hydroxide solution is preferable, in consideration of the hydrolysis speed and foaming during neutralization. The amount of the alkali solution to be added is generally 1-4 equivalent amount, preferably 1.5-3 equivalent amount, relative to hydroxy acid compound (V-4), on conversion to xcex2-hydroxy acid alkali salt compound.
Simultaneously with or before addition of the alkali solution, a solvent such as methanol and the like is preferably added to xcex2-hydroxy acid compound (V-4) for efficient progress of hydrolysis. The amount of the solvent to be added is, for example, generally 0.05-fold volume to 2-fold volume, preferably 0.1-fold volume to 1-fold volume, of the solvent in the solution containing a reducing agent in the above-mentioned Step 1.
The time of hydrolysis is up to the disappearance of xcex2-hydroxy acid compound (V-4), which is generally 1-10 hr, preferably 2-5 hr, and the temperature is generally 0-60xc2x0 C., preferably 10-40xc2x0 C.
The reaction mixture after hydrolysis is isolated and purified by a typical step for treatment and separation. For example, an organic solvent is added to the reaction mixture and the mixture is stirred and partitioned to remove the organic layer, the pH of the aqueous layer is adjusted, the aqueous layer is subjected to extraction with a solvent and concentrated to isolate xcex2-hydroxy acid compound (V-5). The organic solvent to be added after hydrolysis is exemplified by heptane, toluene and the like, preferably toluene. The time for stirring upon addition of the organic solvent is not particularly limited as long as the organic layer and the aqueous layer are thoroughly separated. The pH of the aqueous layer is adjusted to an acidic one (generally 0-3, preferably 1-2) with an acid such as hydrochloric acid, sulfuric acid and the like. The solvent used for extraction of the objective xcex2-hydroxy acid compound (V-5) from the aqueous layer is exemplified by ethyl acetate, a mixture of ethyl acetate-n-butanol (1:1) and the like. The use of ethyl acetate as a solvent is preferable, because the yield becomes high and the objective xcex2-hydroxy acid compound (V-5) can be obtained as crystals. The xcex2-hydroxy acid compound (V-5) obtained as crystals has a comparatively high purity, which is preferable for forming a salt with optically active amine in the next optical resolution step. In addition, extraction is preferably carried out twice.
Production Method of xcex2-hydroxy Acid Compound (V-6) by Optical Resolution of xcex2-hydroxy Acid Compound (V-5)
As shown in the following Scheme 5, xcex2-hydroxy acid compound (V-5) is optically resolved (hereinafter sometimes to be referred to as Step 3) to give S-form or R-form of xcex2-hydroxy acid compound (V-6). For example, by optical resolution by the diastereomeric isomer crystallization method for forming a salt of xcex2-hydroxy acid compound (V-5) with S-form or R-form of the optically active compound, S-form or R-form of xcex2-hydroxy acid compound (V-6) can be obtained. 
Specifically, as an optically active compound reagent for optical resolution, optically active amine is used to give crystals of diastereomer salt, which salt is decomposed (i.e., liberation of amine) to give xcex2-hydroxy acid compound (V-6). The xcex2-hydroxy acid compound (V-5) to be the starting material in this Step 3 can be produced by, for example, the above-mentioned Step 1 or 2.
The optically active compound to be used for this reaction may be any as long as it forms a salt with xcex2-hydroxy acid compound (V-5) and permits optical resolution of xcex2-hydroxy acid compound (V-6) at a high optical purity. Examples thereof include optically active amine compound. Particularly, R-(+)-N-(p-hydroxybenzyl)phenylethylamine of the following formula 
xe2x80x83(R-HBPEA; hereinafter sometimes to be referred to as optically active amine compound (VI)) is preferable, because optical resolution is performed and the aimed optical compound is obtained at a high optical purity in a high yield. The optically active amine compound (VI) in the R-form can be obtained according to the production method disclosed in JP patent No. 3031048. The production example therein is shown in Reference Example below. In the following, Step 3 is explained by referring to an example using an optically active amine compound (VI) as an optically active compound. However, Step 3 is not limited to this example.
To be specific, xcex2-hydroxy acid compound (V-5) is dissolved in the following solvent. The above-mentioned optically active amine compound (VI) is added to this solution, and the mixture is stirred while raising the temperature until the precipitated salt is dissolved, after which the mixture is cooled until the salt is precipitated and stirred. The mixture is gradually cooled and, after stirring further at a given temperature, filtrated, washed with a solvent and dried to give an amine salt of xcex2-hydroxy acid compound (V-6) having a high optical purity.
The solvent in which to dissolve xcex2-hydroxy acid compound (V-5) is exemplified by water; alcohol solvents represented by methanol, ethanol and 2-propanol; ester solvents represented by ethyl acetate; and mixed solvents thereof; and the like. of these, ethyl acetate, methanol, water, and mixed solvents thereof are preferable from the aspect of efficiency of the optical resolution. The amount of the solvent to be used is generally 5-25 L, preferably 7-20 L, relative to 1 kg of hydroxy acid compound (V-5).
The amount of the optically active amine compound (VI) to be used is generally 0.4-1.1 mol, preferably 0.5-1.0 mol, per 1 mol of xcex2-hydroxy acid compound (V-5).
After the addition of optically active amine compound (VI), the temperature of the mixture is generally raised to the boiling point of the solvent or mixture for dissolution of the precipitated salt. When dissolution of the salt is insufficient, the solvent is added until the salt is dissolved. Examples of the solvent include methanol, water, ethyl acetate and the like, preferably methanol and water. The amount of the solvent to be used is generally 0.1-7 L, preferably 1-4 L, per 1 kg of xcex2-hydroxy acid compound (V-5).
For gradual cooling and stirring until the salt is sufficiently precipitated, the temperature of precipitation is generally from the boiling point of the solvent to 0xc2x0 C., preferably 65-10xc2x0 C., and the stirring time is generally 2-12 hr.
The solvent to be used for washing after filtration is preferably a solvent having the composition of the solvent used for precipitation. The amount of the solvent to be used for washing is not particularly limited and is an amount sufficient to thoroughly wash the filtrated product.
Besides the above-mentioned procedure of obtaining optically resolved amine salt, the following procedure can be adopted, i.e., the addition of optically active amine to the solution, temperature raising, stirring and cooling of the solution to obtain optically resolved amine salt, it is possible to gradually allow precipitation of the salt by adding a solution of optically active amine compound (VI) to a solution of xcex2-hydroxy acid compound (V-5) at a temperature of from room temperature to around 40xc2x0 C., and then to allow cooling. For precipitation of the salt, a poorly dissolving solvent (toluene and the like) may be added to a solution containing an acid (xcex2-hydroxy acid compound (V-5)) and amine to allow precipitation of crystals.
Optical Resolution (Recrystallization) of xcex2-hydroxy Acid Compound (V-6)
Recrystallization gives an amine salt of xcex2-hydroxy acid compound (V-6) having a higher optical purity than that of an amine salt of xcex2-hydroxy acid compound (V-6) obtained in the above-mentioned Step 3. To be specific, a salt of the xcex2-hydroxy acid compound (V-6) obtained by the above-mentioned Step 3 is mixed with a solvent to dissolve the salt, and the mixture is gradually cooled. Seed crystals are added at a suitable temperature and the mixture is then gradually cooled and, after further stirring at a given temperature, filtrated, washed with a solvent and dried to increase the optical purity than the amine salt of xcex2-hydroxy acid compound (V-6) obtained in the above-mentioned Step 3.
For recrystallization, a solvent to dissolve a salt of xcex2-hydroxy acid compound (V-6) may be the same as the solvent in which xcex2-hydroxy acid compound (V-6) is dissolved in the above-mentioned Step 3, and preferable solvents are the same. The amount of these solvents to be used for recrystallization is generally 5-20 L, preferably 6-15 L, per 1 kg of a salt of xcex2-hydroxy acid compound (V-6).
The temperature of the solution of a salt of xcex2-hydroxy acid compound (V-6) mixed with these solvents is generally raised to the boiling point of the solvent or mixture, and a different solvent is added until the salt is dissolved. As this solvent, methanol, water and the like are preferable. The amount of the solvent to be used is generally 0.1-5 L per 1 kg of a salt of xcex2-hydroxy acid compound (V-6).
The solution is gradually cooled and stirred after adding seed crystals at a suitable temperature. The stirring time is generally 15 min-3 hr. The reaction mixture is gradually cooled generally to room temperature (35xc2x0 C.)-0xc2x0 C., preferably 20-10xc2x0 C., over 30 min to 12 hr.
The solvent to be used for washing after filtration is desirably a solvent having the composition for crystal precipitation.
The obtained amine salt of xcex2-hydroxy acid compound (V-6) is converted to a xcex2-hydroxy acid compound (V-6) by releasing the amine. The xcex2-hydroxy acid compound (V-6) can be used as a starting material for the next esterification step. The amine salt of xcex2-hydroxy acid compound (V-6) can be decomposed into a xcex2-hydroxy acid compound (V-6) and an optically active amine compound (VI) (R-HBPEA) by a conventional method. For example, the amine salt of xcex2-hydroxy acid compound (V-6) is added to water, a base in an equivalent or more amount and a solvent are added and a partitioned organic layer is removed to separate the amine.
Production Method of xcex2-hydroxy Acid Compound (V-7) Using xcex2-hydroxy Acid Compound (V-6) as a Starting Material
The xcex2-hydroxy acid compound (V-7) is also a novel compound, and can be obtained by esterification of xcex2-hydroxy acid compound (V-6) or a salt thereof with an alkylating agent as shown in, for example, the following Scheme 6. The salt of xcex2-hydroxy acid compound (V-6) is exemplified by those mentioned with regard to the salt of xcex2-hydroxy acid compound (V). 
In the above-mentioned Scheme, R3 is as defined above.
Specifically, the esterification of xcex2-hydroxy acid compound (V-6) or a salt thereof with an alkylating agent (hereinafter sometimes to be referred to as Step 4) varies depending on the use of a free form of xcex2-hydroxy acid compound (V-6) (Method 1) and a salt (e.g., amine salt) of xcex2-hydroxy acid compound (V-6) (Method 2) as a starting material. The hydroxy acid compound (V-6) to be used as a starting material in this Step 4 can be produced by, for example, the above-mentioned Step 1 or Step 3.
Method 1: A free form of xcex2-hydroxy acid compound (V-6) is dissolved in a solvent and an alkylating agent is added for esterification in the presence of a base.
Method 2: A salt of xcex2-hydroxy acid compound (V-6) (e.g., amine salt) and a solvent for extraction are added to water. To the mixture, an equivalent or more amount of a base is added, and the organic layer containing the amine is separated and removed. A solvent is added to an aqueous layer containing xcex2-hydroxy acid compound (V-6), and, for example, an alkylating agent is added for esterification in the presence of a phase transfer catalyst.
In Method 1 and Method 2, the same alkylating agent may be used. The alkylating agent is free of any particular limitation and can be appropriately determined according to the objective xcex2-hydroxy acid compound (V-7). Examples thereof include dimethyl sulfate, diethyl sulfate, methyl bromide, ethyl bromide, allyl bromide, benzyl chloride, benzyl bromide and the like. of these, dimethyl sulfate is preferably used in view of reactivity.
In Method 1, the solvent to dissolve xcex2-hydroxy acid compound (V-6) is exemplified by N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethyl acetamide, acetonitrile and the like, with preference given to DMF in view of the economic aspect and reactivity. The amount of the solvent to be used is generally 2-15 L, preferably 5-10 L, per 1 kg of xcex2-hydroxy acid compound (V-6).
The base to be used along with the alkylating agent in Method 1 is exemplified by potassium carbonate, N,N-diisopropylethylamine, triethylamine, sodium carbonate and the like. Of these, potassium carbonate and sodium carbonate are preferable from the aspect of reactivity. The amount of the base to be used is generally 1-5 equivalent amount, preferably 1-4 equivalent amount relative to xcex2-hydroxy acid compound (V-6), and the amount of the alkylating agent to be used is generally 1-3 mol, preferably 1-2 mol, relative to 1 mol of xcex2-hydroxy acid compound (V-6). The reaction time necessary for the esterification is generally 0.5-8 hr, preferably 1-5 hr.
In Method 2, an amine salt of xcex2-hydroxy acid compound (V-6) is added to water, and a base such as sodium hydroxide, sodium carbonate, potassium hydroxide and the like, a solvent (e.g., toluene, ethyl acetate, preferably toluene) for extracting amine, and the like are added.
The solvent to be added to the aqueous layer containing xcex2-hydroxy acid compound (V-6) in Method 2 is free of any particular limitation as long as it is an organic solvent suitable for extracting xcex2-hydroxy acid compound (V-7) produced by esterification. Examples thereof include toluene, ethyl acetate and the like, preferably toluene.
The phase transfer catalyst to be added for alkylation in Method 2 is exemplified by tetrabutylammonium bromide, benzyltriethylammonium chloride and the like. Of these, tetrabutylammonium bromide is preferable from the economical aspect.
In Method 2, the amount of the alkylating agent to be used is generally 1-3 mol, preferably 1-2 mol relative to 1 mol of a salt (amine salt) of xcex2-hydroxy acid compound (V-6), and the amount of the base to be used is generally 1-4 equivalent amount, preferably 1-3 equivalent amount, relative to a salt (amine salt) of xcex2-hydroxy acid compound (V-6). The reaction time necessary for esterification is generally 0.5-8 hr, preferably 1-5 hr.
In Method 1 and Method 2, xcex2-hydroxy acid compound (V-7) can be isolated and purified from the reaction mixture after the above-mentioned esterification by general post-treatment and separation. In the case of Method 1, for example, an organic solvent is added to the reaction mixture after esterification, pH is adjusted, the solution is partitioned, an organic layer is extracted, an aqueous layer is extracted again with an organic solvent, and the combined organic layers are concentrated under reduced pressure to isolate and purify xcex2-hydroxy acid compound (V-7). In the case of Method 2, for example, an organic layer of the reaction mixture after esterification is extracted, where necessary, an aqueous layer is extracted again, and the combined organic layers are concentrated under reduced pressure to isolate and purify xcex2-hydroxy acid compound (V-7).
Acetonide Form and Optically Active Form of 1.3-diol Derivative and Production Methods Thereof
The present invention further provides a production method of the above-mentioned acetonide form (VIII) or an optically active form thereof of the following formula (VIII) (hereinafter sometimes to be referred to as acetonide form (VIII)), which comprises converting a 1,3-diol derivative of the following formula (VII) or an optically active form thereof (hereinafter sometimes to be referred to as 1,3-diol derivative (VII)) to acetonide as shown in the following Scheme 7. 
The reaction to give acetonide form (VIII) from 1,3-diol derivative (VII) comprises, for example, (A) adding a catalytic amount of an acid catalyst to a solution of 1,3-diol derivative (VII) in 2,2-dimethoxypropane or 2-methoxypropene, (B) adding a catalytic amount of an acid catalyst to a solution of 1,3-diol derivative (VII) in acetone in the presence of a dehydrating agent, or (C) reacting 1,3-diol derivative (VII) with orthoformate and acetone. The respective solvents in the above-mentioned (A)-(C) may be the reaction reagents themselves and may be used in an amount of generally 3-20 parts by weight, preferably 3.5-10 parts by weight, per 1 part by weight of 1,3-diol derivative (VII).
The acid catalyst to be used for the above-mentioned (A) and (B) is not particularly limited and exemplified by p-toluenesulfonic acid, pyridine p-toluenesulfonic acid salt, camphorsulfonic acid and the like. The acid catalyst is used in an amount of generally 0.5-15 parts by weight, preferably 2-15 parts by weight, per 100 parts by weight of 1,3-diol derivative (VII).
The above-mentioned (A)-(C) are carried out at a temperature of generally from 0xc2x0 C. to the boiling point of the solvent.
The dehydrating agent to be used for the above-mentioned (B) is exemplified by anhydrous copper sulfate.
The orthoformate to be used for the above-mentioned (C) is exemplified by methyl orthoformate and ethyl orthoformate and is used in an amount of generally 3-20 parts by weight, preferably 3.5-10 parts by weight, per 1 part by weight of 1,3-diol derivative (VII).
The 1,3-diol derivative (VII), which is a starting material, can be obtained from the above-mentioned xcex2-hydroxy acid compound (V-2) of the following formula 
xe2x80x83wherein R2 is as defined above.
That is, as shown in the following Scheme 8, acetonide form (VIII) can be produced using xcex2-hydroxy acid compound (V-2) as a starting substance via 1,3-diol derivative (VII). 
In the above-mentioned Scheme, R2 is as defined above.
In the production method of acetonide form (VIII), xcex2-hydroxy acid compound (V-2), which is the starting substance, can be produced, for example, according to the method the same as or similar to the above-mentioned xe2x80x9cxcex2-keto ester compound (I) and production method thereofxe2x80x9d and xe2x80x9cxcex2-hydroxy acid compound (V) and production method thereofxe2x80x9d, through xcex2-keto ester compound of the following formula 
xe2x80x83which is xcex2-keto ester compound (I) wherein R4 is hydrogen atom, as shown in following Scheme 9. 
In the above-mentioned Scheme, R1 and R2 are as defined above.
The reduction of xcex2-hydroxy acid compound (V-5) or xcex2-hydroxy acid compound (V-6) to 1,3-diol derivative (VII) is carried out generally using 2 to 8 molar amount of a reducing agent per 1 mol of p-hydroxy acid compound (V-5) or xcex2-hydroxy acid compound (V-6). Examples of the reducing agent include borane, borane complex such as borane-THF complex and the like. Of these, a borane complex (e.g., borane-THF complex) is preferable in view of availability and handling property. As the reaction solvent, ethers such as tetrahydrofuran (THF), 1,2-dimethoxyethane and the like, hydrocarbons such as toluene and the like and mixed solvents thereof are used. The reaction solvent is generally used in an amount of 2- to 10-fold volume amount (2-10 ml) per 1 part by weight (1 g) of xcex2-hydroxy acid compound (V-5) or xcex2-hydroxy acid compound (V-6). The reaction temperature is generally xe2x88x9210xc2x0 C. to 30xc2x0 C., preferably 0xc2x0 C., and the reaction time is generally 1-6 hr, preferably 3 hr.
To be specific, for example, a borane-THF complex is added to a solution of xcex2-hydroxy acid compound (V-5) or xcex2-hydroxy acid compound (V-6) in THF at xe2x88x9210xc2x0 C. to 30xc2x0 C. (preferably 0xc2x0 C.) and the mixture is reacted for 1-6 hr (preferably 3 hr).
The reduction of xcex2-hydroxy acid compound (V-4) or xcex2-hydroxy acid compound (V-7) to 1,3-diol derivative (VII) is carried out using metal hydride generally in an amount of 0.5-5 mol (preferably 1-3 mol) per 1 mol of p-hydroxy acid compound (V-4) or xcex2-hydroxy acid compound (V-7). The above-mentioned metal hydride is exemplified by hydrides of boron and aluminum, such as sodium borohydride, lithium borohydride, diisobutylaluminum hydride and diborane. Of these, sodium borohydride is preferably used from the economical aspect. The reduction is carried out in ethers such as THF and the like, lower alcohols having 1 to 4 carbon atoms such as methanol, ethanol and the like, water and a mixed solvent thereof. The amount of the solvent to be used is generally 5- to 20-fold volume (5-20 ml) amount per 1 part by weight (1 g) of xcex2-hydroxy acid compound (V-4) or xcex2-hydroxy acid compound (V-7). The reduction is generally carried out at a temperature of around 0xc2x0 C. to about 40xc2x0 C., preferably 10-30xc2x0 C. For example, 0.5-3 equivalents (preferably 2 equivalents) of sodium borohydride are added to a mixed solution of xcex2-hydroxy acid compound (V-4) or xcex2-hydroxy acid compound (V-7) in THF-methanol at 0-40xc2x0 C. (preferably around room temperature) and the mixture is reacted for 0.15-5 hr (preferably 3 hr). The completion of the reduction is when the peak of the starting material disappears by detection using gas chromatography and the like.
When a racemate of xcex2-hydroxy acid compound (V-4) or hydroxy acid compound (V-5) is used as a starting substance for the synthesis of acetonide form (VIII) through the above-mentioned 1,3-diol derivative (VII), a reagent for optical resolution as used in the above-mentioned Step 3 is used at some stage in the aforementioned reaction, thereby to obtain an optically active acetonide form alone from the racemic intermediate. This optical resolution may be conducted at any stage, but xcex2-hydroxy acid compound (V-6) or xcex2-hydroxy acid compound (V-7), which is an optically active form, is preferably used as a starting substance in view of operability of the optical resolution. Preferably, xcex2-hydroxy acid compound (V-6) or xcex2-hydroxy acid compound (V-7) is subjected to the synthesis of 1,3-diol derivative (IX), as mentioned earlier, to give an optically active 1,3-diol derivative of the following formula 
xe2x80x83and this optically active 1,3-diol derivative is converted to an optically active acetonide form of the following formula 
The acetonide forms (VIII) and (VIII-1) of the present invention can be used as a constituent factor of the portion enclosed with a broken line in the following formulas showing the epothilone derivatives according to various conventionally known methods. 
Using xcex2-hydroxy acid compound (V-1) of the following formula 
xe2x80x83wherein R4xe2x80x2 is alkyl group having 1 to 6 carbon atoms, or an optically active form thereof, which is a compound of the formula (V) wherein R4 is alkyl group (R4xe2x80x2) having 1 to 6 carbon atoms and R2 is tert-butyl group, acetonide form may be produced via a 1,3-diol derivative in the same manner as in the above-mentioned, as shown in the following Scheme 10. 
In the above-mentioned Scheme, R4xe2x80x2 is as defined above.
The acetonide form of the above-mentioned formula (VIIIxe2x80x2), which is obtained in this way, is considered to be useful as a synthetic intermediate for an epothilone derivative.
The present invention is described in more detail in the following by means of Examples and Reference Examples, which are not to be construed as limitative.